1. Field of the Invention
The present invention relates to a hypersensitive photodetection device in which a CTIA (capacitive trans-impedance amplifier) or CIA (capacitive impedance amplifier) is applied to the detection of feeble incident light such as is found in fluorescence observation in chemistry and weak-light observation in astronomy.
2. Description of the Related Art
The sensitivity of infrared-ray sensors, i.e. sensors used to detect infrared rays, is increasing as semiconductor infrared-ray sensors and related peripheral-device techniques evolve. Two-dimensional infrared-ray sensor elements with detected noise levels of ten-odd electrons have been developed.
The detection noise of two-dimensional infrared-ray sensors has been decreased through reduction of the noise occurring in the detecting MOS FETs (metal oxide semiconductor field effect transistors), and their input capacitance and leakage current have been lessened, as well as through improvements in the performance of the two-dimensional infrared ray sensor. That is, for a given amount of photocurrent in the photodetector, a smaller input capacitance leads to a higher input gate voltage, and less leakage current leads to lower levels of shot noise.
MOS FETs are used in the readout circuits of two-dimensional infrared-ray sensors because of the small characteristics-dispersion, leakage current, and input capacitance of a MOS FET. However, the noise level of a Si J FET (silicon junction field effect transistor), is about 1/100 that of a MOS FET, while its input capacitance is in the range from a few to ten times as high.
On the whole, however, the low-noise Si JFET is expected to improve the S/N over that for a MOS FET. Actually, if we only consider the currently known forms of noise in a Si J FET, such as thermal noise and generation-recombination noise etc., we would expect to be able to obtain measured noise at the single-electron level in the band around the 10-Hz range.
However, success in the measurement of such a noise level has not been reported. Contradicting our expectations, one report stated that the low noise level seen when a J FET is used in an amplifier that has a low input impedance becomes several times higher when it is used in an amplifier with a high input impedance. This noise level is not explicable in terms of the shot noise of the leakage current. The increase in noise has been vaguely assumed to be because the storage noise is strengthened by the increase in input impedance.
Studies by the inventor of the present invention have shown that the dielectric polarization noise is dominant in the high input impedance case. The polarization noise is caused by the phenomenon of the thermal fluctuation of polarization, which is derived from the fluctuation-dissipation theorem as well as Johnson noise is derived from the theorem.
The polarization noise is the principal limit on the noise, because this noise is inevitable in a photodetector and J FET used as a sensor, that is, the noise is inherent to the materials. The detected noise of the prior-art photodetection device has not approached the theoretically limiting range of noise levels because of noise generated by the leakage current or and other noise sources of the FET.
FIG. 7 shows an example of the circuit for a photodetector of the prior art. A CTIA circuit, which is an orthodox circuit and in generally use in photodetection devices, is used in the photodetection device of the prior art shown in FIG. 7.
The CTIA circuit is a TIA (Trans-Impedance Amplifier) in which a capacitor replaces a resistor of the feed-back circuit. A resistor induces thermal noise which determines the limit on the detection of light, but the capacitor in the feed-back loop induces little thermal noise and improves the limit on the detection of light.
However, the photocurrent does not disappear outside the circuit, and charges up the capacitor. We thus need to evacuate the accumulated charge with appropriate timing. This action is called a reset, while the step of measuring the photocurrent is called carrier accumulation (charging of the capacitor).
The photo detection circuit consists of the photodetector that detects incident light (for example, a photodiode) 1, a J FET 2 for reading out the detected light the detection of light, an op amp. (operational amplifier) 3 that amplifies the detection signal, the feed-back loop that feeds the output of the op amp. 3 back to the gate of the J FET 2 through the capacitor 4, and the reset circuit that resets the capacitor 4 by discharging the capacitor 4 through a MOS FET, S1.
Furthermore, the photodetector (for example, a photodiode) 1, input J FET 2 for readout, capacitor 4 and MOS FET S1 are placed in a cryogenic vessel which is cooled down to a cryogenic temperature (for example 77 K), while the amplifier 3 is placed at room temperature.
In this case, the elements in the cryogenic vessel are connected with the op amp. 3 by conductors. Also, the reset pulse to reset the MOS FET S1 can be applied with a control circuit (not shown in the figure) situated outside the vessel, with conductors connecting the MOS FET S1 and this control circuit.
In the above circuit, a feed-back capacitor 4 replaces a resistor of the TIA circuit which is generally used in infrared-ray readout circuits. To give the TIA circuit a large S/N, as large a resistance value as is possible is chosen for the resistor of the overall TIA circuit's feed-back circuit. Johnson noise is thus the dominant form in which noise is generated, and this prevents the detection of infrared rays. When the feed-back capacitor 4 is used in the feed-back loop, the photocurrent provides charge that accumulates in the feed-back capacitor 4, so the CTI circuit is provided with the MOS FET 1 for resetting as a reset switch that discharges the feed-back capacitor 4.
Furthermore, since a very high input impedance is needed to detect the weak radiance of infrared rays, an FET is used in the input circuit. An FET can operate at low temperatures, and can thus be placed very close to the cooled sensor; that is, the length of the high-impedance portion is wired to a short length. While the op amp. is placed at room temperature, all devices other than the OP amp. are set in the cryostat of liquid nitrogen.
The circuit contains noise sources of various kinds that originate in the device elements. However, if we consider the behavior of the circuit, the noise sources can be classified into two types. One type covers the noise voltages generated in the source circuit of the J FET 2, which is the input FET. This noise includes all noise generated in the channel of the J FET 2 and the input-referred noise of the OP amp. Both noise voltages and noise current are referred to as input noise of the OP amp.
However, as the current noise is converted to voltage by the output impedance of the input J FET 2 for readout (henceforth referred to as the input J FET), the current noise can be included with the voltage noise. The noise when converted to current noise at the gate of the input J FET 2 is to be compared with the photo current. The input-referred noise current to the input J-FET can be obtained by dividing the noise voltage with input impedance of the J-FET. S/N of the photodetctor is measured from comparing the photocurrent of the photodetector with the input-referred noise current.
The other kind of noise is that which flows directly into the gate circuit of the input J FET 2, for example the shot noise of the leakage current of the photodetector and the input J FET 2 for readout, and gate-induced noise of the JFET 2. The polarization noise of devices connected to the gate circuit is also of this kind. Noise of this kind is converted to input current noise, and is thus referred to as gate current noise.
The respective two kinds of noise mentioned above can be measured by using the dependence of input impedance. The noise is converted to the referred noise voltage to op amp. 3 output by multiplying the feed-back impedance to the input referred noise current. Thus referred noise voltage of the source noise to op amp. 3 output is proportional to the ratio of the input impedance to the feed-back impedance, while the current noise at the gate is proportional to the feed-back impedance alone.
Thus, lowering the input impedance and feed-back impedance such that the ratio is kept constant reduces the current noise at the gate to negligible levels. On the other hand, when both of the impedances are increased, the current noise returns to measurable levels.
In the prior-art photodetection device, the MOS FET S1 is connected in parallel with the feed-back capacitor 4. In this connection, the voltage between the electrodes of the feed-back capacitor 4 is applied directly across the source and drain of the MOS FET S1 , and this induces a leakage current between the source and drain of the MOS FET S1.
Thus, even when the MOS FET S1 is off, the flow of some leakage current between the source and drain is inevitable whenever any voltage is applied across them. Thus, the noise is not reducible in spite of the use of the feed-back capacitor 4 in the photodetection device; this prevents high sensitivity in photodetection.
When a photodetector that has a larger light-incident area is used so that more light is received, the capacitance of the photodetector inevitably increases. On the other hand, the limit on the sensitivity of photodetection is determined by the noise level of the input J FET that reads out the detection signal. The input-referred noise of the input J FET is proportional to the capacitance of the photodetector; however, the polarization noise is proportional to the square root of the capacitance. Thus, as the capacitance of the photodetector is increased, the input-referred noise of input J FET becomes dominant. Reducing the noise level to the limit imposed by polarization noise is thus difficult with the prior-art photodetection device.